1. Field of the Invention
The present invention relates to a signal processing apparatus and a non-destructive testing apparatus using the same. More particularly, the present invention relates to a signal processing apparatus and a non-destructive testing apparatus using the same, in which the difference between a signal component and a noise component in spatial frequency component is used for noise reduction.
2. Description of the Related Art
Pipes with various diameters are used in a large-scaled plants such as atomic generation plants and thermal power generation plants. These pipes are subject to influences such as vibration and heat change. As a result of these influences, flaws or damage sometimes occurs inside the pipe. In order to ensure the safety of the operation of the large-scaled plant, the pipes need to be tested to see if any flaw or damage has occurred. A non-destructive test is usually performed.
There are various conventional non-destructive test methods. Known general non-destructive test methods include a supersonic flaw detecting method and an eddy current testing method.
In the eddy current test (ECT) method, small flaws can be detected, however, the results can be influenced by noise. A sensor used in the eddy current test is generally a rotary type sensor, which is rotated in a circumferential direction in a test body, such as a pipe, while progressing in an axial direction into the test body. Eddy current is generated in a metal body of the test body by a moving magnetic field generating body or a stationary magnetic force changing coil. The magnetic force generated by the eddy current is measured as a function of a position coordinate.
A noise component comprises a support structure noise component, a pipe diameter change noise component, an adhesion noise component, a sensor fluctuation noise component and an electric noise component. Techniques such as (1) a band pass filtering method, (2) a multiple frequency calculating method and (3) a line filtering method are conventional methods that are used to remove these noise components.
In the band pass filtering method, a signal is observed from the test body, such as a pipe, as a time series signal, and a component of the signal, other than a specific frequency band, is attenuated. This band pass filtering method will be described with reference to FIG. 1. First, in order to analyze in a frequency region, a observation signal x(n), as the time series signal, is converted (Fourier transform) from a time space into a frequency space so that a frequency spectrum X(f) is obtained. The observations signal X(n) comprises a noise component and a damage component, as a detection object component, which indicates the existence and shape of any damage in the test body.
Next, a weighting operation is performed on the frequency spectrum X(f) by using a band pass window having the frequency response of w(f) to obtain a frequency spectrum Xxe2x80x2(f). The weighting operation attenuates the frequency component, other than the specific frequency region. In order to obtain a band pass signal Xxe2x80x2(n) after the filtering, an inverse Fourier transform is performed to the band pass signal Xxe2x80x2(f) to obtain a band pass signal Xxe2x80x2(n) as a time series signal in which the frequency component, other than the specific frequency band, is attenuated.
FIG. 2 shows the multiple frequency calculating method. In this method, the way of changing a detected signal is different, depending on a signal generation factor, when an excitation frequency is different. In the multiple frequency calculating method, a linear calculation of a multiple frequency signal (X1 (t), X2(t), X3(t) and X4(t)) is performed by using filtering parameters (W1, W2, W3 and W4) previously set to output a synthetic signal Y(t) in which only a damage component as a detection object component remains.
FIGS. 3A and 3B show the line filtering method. In this method, a component for one line specified as a reference line is removed from a 2-dimensionally distributed original signal.
In the above-mentioned band pass filtering method, the frequency spectrum of the observed time series signal is analyzed by paying attention to 1-dimensional component of the 2-dimensionally distributed signal. Therefore, the detected frequency response is easy to change depending upon the directions of the detection object component and noise component.
Another problem is that the signal obtained after the filtering operation looks like vibration. Therefore, the position precision is degraded when attempting to narrow the band width of the filter.
The multiple frequency calculating method is effective when the phase angles of the of the detection object component and noise component are clearly different. However, when the phase angles are close to each other, the filtering effect is low. For example, phase angles are close to each other when there is a damage signal and a deformation signal on the surface of the test body on side of the sensor.
Moreover, the line filtering method reduces a uniformly distributed noise component in a 1-dimensional direction of the 2-dimension. However, the reduction is minimized when there is a noise component other than the uniformly distributed noise component in the 1-dimensional direction. Also, even if the noise component is uniformly distributed in the 1-dimensional direction, an unnecessary component remains when the uniformity is not broken due to the drift and so on. Moreover, the reference line as the main point of the line filtering method should be estimated based on the observation signal. Therefore, there is a risk that the detection object component will be attenuated when a mistake is present in the estimation.
In the eddy current testing method, the properties of the damage such as direction, length, width and depth have various values. However, because a detection signal is obtained by observing the change of the eddy current flowing through the test body, the frequency components of the detection signal are spread 2-dimensionally in accordance with the excitation frequency, even if the damage is small.
On the other hand, the noise component is different from the damage component as a detection object component in a 2-dimensional spatial frequency spectrum. The noise component comprises the pipe support structure noise component, the pipe diameter change noise component, the adhesion noise component and the sensor fluctuation noise component. The noise component has a low frequency component in at least one of the 2 dimensions, as compared with the spatial frequency spectrum of the damage component. On the other hand, the electric noise component has a frequency component higher than the spatial frequency spectrum of the damage component.
An object of the present invention is to provide a signal processing apparatus which has a highly precise detection ability, and in which the filtering technique is used based on the difference between a detection object component and a noise component to be attenuated in a spatial frequency spectrum.
Another object of the present invention is to provide a non-destructive testing apparatus using the above signal processing apparatus.
In order to achieve an aspect of the present invention, a signal processing apparatus includes a 2-dimensional interpolation processing section, a 2-dimensional emphasis and attenuation processing section and a 2-dimensional smoothing processing section. The 2-dimensional interpolation processing section maps an observation signal in a first coordinate system into a second coordinate system to output a 2-dimensional interpolation signal. The observation signal includes a detection object component and a noise component to be attenuated, and the noise component is composed of a first noise component and a second noise component. The 2-dimensional emphasis and attenuation processing section attenuates the first noise component to emphasize the detection object component and outputs a 2-dimensional filtering signal in which the detection object component is emphasized. The 2-dimensional smoothing processing section attenuates the second noise component contained in the 2-dimensional filtering signal, and outputs a 2-dimensional smoothing signal, whereby a detection object can be detected based on the 2-dimensional smoothing signal.
The 2-dimensional emphasis and attenuation processing section may include a 2-dimensional digital differential filter. Also, the 2-dimensional smoothing processing section may include a median filter.
In order to achieve another aspect of the present invention, a non-destructive testing apparatus includes a detector, a display unit and a processor. The detector measures a test object and generates a measurement signal in a first coordinate system. The processor maps the measurement signal in the first coordinate system onto a second coordinate system to produce a second coordinate system measurement signal, removes a noise component from the second coordinate system measurement signal to produce a resultant signal, and controls the display unit to display the resultant signal.
The detector may be a rotary type detector, wherein the first coordinate system is a polar coordinate system, or a multi-coil type sensor, wherein the first coordinate system is a 2-dimensional coordinate system.
Also, the second coordinate system may be a 2-dimensional orthogonal coordinate system. In this case, the processor converts each of the values of the measurement signal to a value on the 2-dimensional orthogonal coordinate system, while mapping the measurement signal in the first coordinate system onto the 2-dimensional orthogonal coordinate system.
Also, the processor may attenuate a part of the noise component from the second coordinate system measurement signal. In this case, the processor may attenuate the part of the noise component from the second coordinate system measurement signal using a first frequency cutting type filter. In addition, the filter may be a 2-dimensional digital differential filter.
Also, the processor may remove the remaining part of the noise component from the second coordinate system measurement signal. In this case, the processor smoothes the second coordinate system measuring signal to remove the remaining part of the noise component from the second coordinate system measurement signal. Moreover, the processor smoothes the second coordinate system measuring signal using a filter to remove the remaining part of the noise component from the second coordinate system measurement signal. The filter may be a median filter.
In order to achieve still another aspect of the present invention, a non-destructive testing method of a test object includes:
measuring a test object to generate a measurement signal in a first coordinate system;
mapping the measurement signal in the first coordinate system onto a second coordinate system to produce a second coordinate system measurement signal;
removing a noise component from the second coordinate system measurement signal to produce a resultant signal; and
providing information of the test object based on the resultant signal.
In order to achieve yet still another aspect of the present invention, a non-destructive testing apparatus includes:
a detector measuring a test object to generate a measurement signal in a first coordinate system;
a display unit;
a first filter that attenuates a first frequency region of an input signal to produce a first filtered resultant signal;
a second filter that attenuates a second frequency region of an input signal to produce a second filtered resultant signal, the second frequency region being apart from the first frequency region; and
a processor that:
maps the measurement signal in the first coordinate system onto a second coordinate system adaptable for the first filter,
selectively maps a first filtered resultant signal in the second coordinate system into a third coordinate system adaptable for the second filter, and
controls the display unit to display a second filtered resultant signal.